1. Field of the Invention
The present invention relates generally to valves and, more particularly, to a liquid barrier packing system adapted for integration into a valve and operative to satisfy very low fugitive emission leakage standards.
2. Description of the Related Art
In a typical valve construction, a valve stem may undergo a turning or sliding movement, or a combination of both movements, within its sleeve during the process of the valve moving between its open and closed configurations. In this regard, the sealing of the stem must be adequate to contend with such movement, while at the same time ensuring maintenance of fluid tightness against the pressure of the fluid flowing through the valve. A widely used type of stem sealing is a compression packing in which a gland or sleeve is used to apply a compressive force to a compression packing which surrounds a portion of the length of the stem. The resulting radial pressure of the packing onto the stem provides the desired seal so long as the radial pressure exceeds the pressure of fluid in the valve.
In certain valve configurations, compression may be applied to the packing through the use of packing bolts which are each attached at one end to a valve bonnet of the valve, and at their other end to a spigot, a flange or other projection bearing on, integral with or attached to the gland or sleeve which bears onto the packing. In this particular arrangement, the tightening of the bolts increases the pressure on the packing, thus facilitating the application of radial pressure onto the stem. In other valve configurations, it is known to attach a spring between the nut used to tighten the bolt and a surface of the spigot or flange. Although coil springs may be used, a conventional practice is to use Belleville springs which are essentially formed as a series of dished washers. These Belleville springs provide a “live-loaded” packing which can automatically compensate for changes that may take place in the packing under operating conditions of the valve, such as high pressures and temperatures. Since the volume of the packing material may reduce under certain operating conditions, or the temperature increase of the bolts and their further elongation may result in a load loss, the spring pressure compensates for such reduction and maintains the required pressure, thus avoiding potential harmful effects to the sealing of the stem in an unsprung valve which could result from the reduction in the packing material volume. Alternatively, if the volume of the packing material increases (which can happen with certain packing materials), the radial pressure of the stem in an unsprung valve could increase too much, thus possibly causing sticking of the stem. The spring value, however, can accommodate the pressure increase by means of further compression of the springs.
Recently, there has been an increasing level of demand in many oil and gas applications for the low level emission of Volatile Organic Compounds (VOC's). In this regard, in a typical oil and gas production and processing plant, control valves are generally considered to be the largest contributors to the loss of VOC's. This has resulted in the owners of many of these facilities developing strict fugitive emission specifications to minimize VOC leakage attributable to the valve stem packing, with allowable valve stem packing leakage rates being very low. Additionally, various laws enacted in Europe and other jurisdictions currently define the maximum concentration level of pollutants that can be detected in the air in an industrial setting, and proximate valves located therein. These laws and regulations are having the effect of forcing valve manufactures to adopt new designs for valve packing and sealing systems to comply with the same.
However, the packing system included in many valve designs, including those which include a live-loaded packing as described above, is still often susceptible to varying levels leakage about the valve stem. Though some solutions have been developed which make use of a barrier fluid, these particular solutions do not provide a live loaded system to maintain the barrier fluid pressure at a level higher than that of the process pressure, thus diminishing the longevity of the packing integrity once in service conditions (see, e.g., U.S. Pat. No. 7,118,114). In one existing barrier fluid solution, grease is laterally injected into a valve bonnet. However, in this particular solution, the grease is typically lost after repeated valve cycling, with its efficacy as a fluid barrier thus only being somewhat temporary unless replenished on a frequent basis. As will be recognized, a loss of efficacy of the grease as a fluid barrier prior to replenishment may result in undesirable leakage. Further, the attempted replenishment of the grease while the valve is still pressurized can jeopardize the integrity of the valve packing, thus creating a potential hazard to operators if high pressure gas escapes the valve bonnet. In addition, the use of the aforementioned lateral injection technique gives rise to the potential for lateral grease escape during the operation of the valve, thus creating a possible leak source. Still further, the aforementioned solution, as currently known, lacks modalities for detecting when the grease level is falling to an ineffective level. Other solutions are relatively complex to manufacture, assemble and service. Further, the existing solutions typically ignore the role of a seal to shaft interface in friction and seal wear, and the resultant impact on leakage levels.
The present invention addresses the problem of packing leakage as it relates to VOC's by providing a low diffusivity barrier fluid or liquid packing system which is configured to be accommodated by a traditional valve stuffing box, and is further adapted to minimize VOC emissions, while also providing live-loading and continuous load monitoring functions. These, as well as other features and attributes of the present invention will be discussed in more detail below.